In hard disk drives, a magnetic head mounted at tip portion of an actuator arm is moved using the actuator arm. The magnetic head is moved in radial directions of a magnetic disk that is a recording medium to record and read data onto and from the magnetic disk. The recording medium has data areas onto which to record user data and servo areas on which data used in head positioning control is recorded.
In the above hard disk drive, when the magnetic head passes across the servo area, position of the magnetic head is identified based on a signal read from the servo area. And in the hard disk drive, by adjusting the position of the magnetic head based on the identified position by the actuator arm, the magnetic head can be controlled to be in a desired position.
As to patterns recorded on the servo area, there are several types, for example, a NULL servo pattern, an area servo pattern, a phase servo pattern, and the like exist. For example, the NULL servo pattern comprises two blocks different in magnetization pattern. The position of the magnetic head can be identified by position signals read from the two blocks of the NULL servo pattern. It is understood that these position signals are represented as a Lissajous figure when they are plotted on a phase plane, and that as its shape becomes closer to a circle, the position can be identified more accurately.
However, various distortions occur in the actually obtained position signals. For example, the Lissajous figure may be not a circle but deformed in a rectangle shape or inclined. Thus, in the conventional art, improvement in accuracy in identifying the position of the magnetic head is attempted by correcting for the above deformation. However there is some room for improvement in the method of determining the amount of correction.